1. Field of the Invention
The present invention pertains to hardness testing apparatus. Specifically, it pertains to hardness testing apparatus of the type having a load cell by which predetermined forces can be transferred to a penetrator for measuring hardness properties of materials contacted thereby in response to loads applied through the load cell. Specifically, the invention pertains to operating cams for use with load cell type hardness testers and by which no load, minor load and major load forces, to be applied to the penetrator, are controlled.
2. Brief Description of the Prior Art
Load cell type hardness testers conventionally comprise a housing in which is carried a load cell having major and minor load springs therein. The tester includes a penetrator and a plunger rod by which the load forces of the load cell are transferred to the penetrator. An operating cam is normally provided which in certain positions prevents all or part of the load forces of the load cell from being transferred to the penetrator. The operating cam is normally provided with a handle, rotation of which allows predetermined no load, minor load or major load forces to be applied to the penetrator to indent the material being tested. Depending upon the load transferred from the load cell, the plunger rod and penetrator respond in a predetermined fashion to the indentation of the penetrator in the material being tested. A dial gauge is attached to the tester for engagement with the opposite end of the plunger rod, measuring the response thereof, to determine the hardness of the material being tested.
In hardness testers of the prior art, the operating cam is provided with first and second cylindrical journal portions for disposition within first and second cylindrical holes of the tester and an intervening cylindrical cam portion of a smaller diameter which is eccentric to the axis of the journal portions. One end of the operating cam is provided with a handle or some other means of rotating the cam and the opposite end of the operating cam is provided with a stop member, usually in the form of a pin member which extends from the first cylindrical journal portion. The pin member engages corresponding stop shoulders on the housing of the tester in the no load and major load positions. However, this arrangement creates certain problems.
Due to the relatively large torque which may be applied to the stop pin, the stop pin is frequently sheared off. This is due to the relatively large ratio between the radius at which the operating force is applied to the handle and the relatively small radius of the operating cam journal at which the stop pin is disposed. In addition, the stop pin is usually small in diameter due to the fact that holes for larger stop pins would weaken the operating cam journal. When a stop pin is sheared it must be replaced, sometimes requiring disassembly of the tester.
Another problem with operating cams of the prior art, as just described, is the distortion of the eccentric cam portion. Since it is of smaller diameter than the journal portions of the operating cam and since the stop pin is at an end of the operating cam opposite the handle, the eccentric cam portion must absorb and transfer the torque created by the force applied to the operating handle with the pin in either the no load or major load stop positions. Consequently, the intermediate eccentric cam portion may be twisted or distorted so that an improper load is transferred to the tester penetrator, resulting in inaccurate measurements. Furthermore, if too much torque is applied, it may even result in twisting the operating cam in two.
Thus, it is seen that the operating cam of load cell type hardness testers of the prior art are high maintenance items. Not only does such maintenance result in increased repair costs but it also results in prolonged testing time.